The invention relates to color video display terminal (VDT) devices that employ cathode ray tube (CRT) screens. Typically, such CRT's involve three primary color cathode ray guns which are manipulated to converge on a screen that produces the color image. The three guns produce converged scanning rasters having red, green and blue fields which combine to produce white light. The typical scanning raster is an interleaved left to right horizontal and top to bottom vertical scan operated in accordance with the National Television Standards Committee (NTSC) requirements adopted by the FCC for commercial color television.
Each of the three CRT guns must be supplied with the correct video signal and DC bias controls so that the required image will appear on the screen. Typically, a CRT cathode is supplied with a 60-volt video signal. (It is to be noted that where signals are to be described in this specification, the numbers will be in terms of peak-to-peak values.) A video amplifier is incorporated with a preamplifier and video driver to produce a signal gain of about 120 so that a one-half volt video input is required. Each of the three required amplifier channels will have individual gain controls which act as vernier drive controls. These controls have a relatively limited range of control and can be set to accommodate the characteristics of the particular CRT guns involved. This is necessary because the manufacturing tolerances of the CRT result in different control voltage requirements. Once the drive controls are set in the three channels they will compensate the individual gun characteristic differences and will not need to be adjusted further. In addition, the three channels will have a common gain control which determines the picture contrast. Once the three drive controls are set initially, when the contrast control is varied, the three channels should track each other.
Each of the three channels should have a cutoff bias adjustment which sets the DC level at the CRT cathode to define the black reference level. This adjustment, which is also related to the CRT characteristics, is also set once to allow for the CRT manufacturing tolerances. The three channels also have a common DC bias control that permits setting the DC cathode voltages unison to the desired reference or "black level" and, therefore, functions as a picture brightness control. Once the individual cutoff controls are adjusted the three guns should track each other in response to the brightness control setting.
Still another DC level control, called the cutoff adjust, is common to the three channels. It acts to set the blanking level of the three guns below the so-called black reference level of the display. This assures a "blank" VDT screen during the blanking interval. The video amplifier normally includes a DC restoration circuit that automatically adjusts the DC signal level so that the blanking signal level in the composite video is established as the reference level. Thus, if the three guns are biased to cutoff at the blanking level position, signal values are seen as picture elements and the negative synch pulses are not presented. This means that an input AC coupled signal is clamped so that the DC level is referenced to the blanking pulse and a DC reinsertion is thereby accomplished. With respect to the composite video, this is done by means of a clamp circuit that is gated on just after the trailing edge of the horizontal sync pulse. Thus, the gate interval exists upon the back porch of the horizontal blanking pulse of the composite video. During this gate pulse interval the video DC level is sampled and a capacitor charged to the sample value. The capacitor charge is applied to the video amplifier thereby providing a DC bias level that persists during the interval between sample pulses. Thus, as the composite video levels change, the DC level during blanking is held constant and the desired DC insertion is achieved.